The invention relates to an optical device.
There is currently considerable interest in the use of high-reflectivity grating filters for use as feedback and filtering elements in, for example, fibre lasers. At present, the generation of high reflectivity grating filter involves etching gratings formed in photoresist on top of polished directional couplers and providing an overlying layer of oil with a refractive index matching that of the underlying waveguide. An example of this is described in "High-Reflectivity Monomode-Fibre Grating Filters" Electronics Letters, 13th March 1986, Vol. 22, No. 6, pages 341-343.
The generation of these filters involves complex fabrication procedures which are difficult to circumvent and expensive.